This invention relates to a static pressure table device, and more particularly, to a static pressure table device suitable for use as a slide bearing of a large size precision machine tool.
In large size machine tools, a static pressure table device has been used as a means for rotatably supporting a rotary table on a bed. In this table device, static pressure oil is supplied between the contact surfaces of the bed and the table, the oil acting as a medium for supporting the rotary table. In this device, pressurized oil is supplied to a gap between the contact surfaces for providing a load supporting function which is used for rotatably supporting the rotary table so as to effect a precision finish machining of a workpiece mounted on the rotary table.
For suppressing temperature variation as small as possible, it has been the practice to prevent an influence of generated heat by controlling the temperature of the static pressure oil, the room temperature and the machine temperature or to form an annular recess or recesses in the contact surface of the bed and supply a static pressure oil into the recess or recesses in a manner to enable pressure changes in the static pressure oil.
As a reference describing a static pressure lubrication theory of a slide bearing utilizing static pressure oil, a publication is known which is entitled "static pressure lubrication theory" and published Feb. 9, 1968 by Japan Institute of Lubrication (Nihon Junkatsu Gakkai) (Editor: Yoshiro Mori).
The static pressure table device (slide bearing) described above is used mainly for a ultra-high precision machine tools or machine tools for machining a workpiece of large size and heavy weight. For this reason, shearing heat is generated in the static pressure oil in the gap between the bed and the rotary table due to shearing forces produced by the relative motion therebetween, and the shearing heat directly affects the machining accuracy. Particularly, because the temperatures due to the heat generated in the manner described above differ at the central portion and the peripheral portion of the rotary table, the rotary table undergoes thermal deformation (deflection) as in the case of a bimetal. More specifically, where the oil is supplied from the stationary side (bed side), a portion of the shearing heat in the oil is transmitted to the rotary table side, causing a temperature difference between the bed and the rotary table so that there occurs deformation of the rotary table having a smaller heat capacity than the bed. As a consequence, the static pressure gap varies, with the result that high speed rotation of the table becomes impossible. For this reason, in case where thermal deformation due to high speed rotation occurs, it was necessary to rotate the table at low speed for enabling precision machining.
If, for the purpose of suppressing the heat generation in the static pressure gap, the static pressure oil is supplied after lowering its temperature, the temperature at the central portion of the rotary table will decrease too much so that a temperature difference will be created between the central portion and the peripheral portion of the table and a thermal deformation of the table will inevitably occur.
While it is necessary to form an annular recess on the upper surface of the bed for the purpose of compensating for elastic deformation of the bed and of maintaining static oil pressure when the elastic deformation occurs, the amount of deformation is large in the case of a large size machine tool, and in this regard it is necessary to use a pump of a large capacity for supplying the static pressure oil. This large capacity oil supply pump has a disadvantage in that it requires a large energy as well as a large amount of cooling system.
The reference cited above describes analyses of the pressure of the lubricating oil, but does not refer to the technique regarding heat so that this reference does not suggest any solution of the problem caused by heat.